This invention relates to methods and apparatus for controlled engagement and disengagement of the clutch in the transmission of a motor vehicle drive system having an electrohydraulically actuatable automatic transmission and, more particularly, to a method and apparatus for reducing stationary vibration of the vehicle when in gear with the drive wheels braked and for reducing concussion during engagement and/or disengagement of the drive system.
When a motor vehicle having an automatic transmission is stationary and in gear with the drive wheels braked, so-called stationary vibration occurs. The cause of this vibration, which is disagreeable to the operator of the vehicle, is the opposition between the drive system, which produces a propulsive output, and the body of the vehicle, which receives this output but cannot pass it on to the wheels. This condition of the drive system is thus defined by the interrelationship between the torque of the drive system and the reaction torque of the vehicle body.
In prior art drive systems, such stationary vibration is avoided when the vehicle is stationary, in gear and the drive wheels are braked, by fully disengaging the clutch of the engaged gear so that there is no strain between the body and the drive system. In order to initiate driving from this condition, the slack in the clutch must first be taken up. In this time interval, during which the clutch is engaged but has not yet taken hold, the engine is free to race. Then, when the clutch finally takes hold, the engine speed has been elevated to produce an excessive output torque and the operator experiences a disagreeable engagement concussion. This makes clear that, by completely opening the clutch to avoid stationary vibration, another component of driving comfort has simply been traded off.
Both stationary vibration and engagement concussion are especially disadvantageous in drive systems having diesel engines. In such drive systems, a mechanical idling-speed governor normally maintains the engine at a relatively high minimum rotational speed. When the vehicle is braked but the clutch is not disengaged, this high engine speed and the comparatively hard action of the engine provide especially intense stationary vibrations. The same comparatively high idling speed of the diesel engine, however, also results in an especially hard engagement concussion when a previously disengaged clutch is engaged.